1. Field of the Invention
The present invention relates to a write head before read head constructed merged magnetic head with the track width and the zero throat height of the write head being defined by a first pole tip and more particularly to a magnetic head wherein a first pole tip of a first pole piece is frame plated on a planarized surface so as to accurately define the track width of the write head with high resolution and wherein planarization after constructing the first pole tip reduces separation between read and write gaps of the magnetic head.
2. Description of the Related Art
An inductive write head includes a coil layer embedded in first, second and third insulation layers (insulation stack), the insulation stack being located between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a gap layer at an air bearing surface (ABS) of the write head. The pole piece layers are connected at a back gap. Currents are conducted through the coil layer, which produce magnetic fields in the pole pieces. The magnetic fields fringe across the gap at the ABS for the purpose of writing bits of magnetic field information in tracks on moving media, such as in circular tracks on a rotating magnetic disk or longitudinal tracks on a moving magnetic tape.
The second pole piece layer has a pole tip portion which extends from the ABS to a flare point and a yoke portion which extends from the flare point to the back gap. The flare point is where the second pole piece begins to widen (flare) to form the yoke. The placement of the flare point directly affects the magnitude of the magnetic field produced to write information on the recording medium. Since magnetic flux decays as it travels down the length of the narrow second pole tip, shortening the second pole tip will increase the flux reaching the recording media. Therefore, performance can be optimized by aggressively placing the flare point close to the ABS.
Another parameter important in the design of a write head is the location of the zero throat height (ZTH). The zero throat height is the location where the first and second pole pieces first separate from one another after the ABS. ZTH separation is imposed by an insulation layer, typically the first insulation layer in the insulation stack. Flux leakage between the first and second pole pieces is minimized by locating the ZTH as close as possible to the ABS.
Unfortunately, the aforementioned design parameters require a tradeoff in the fabrication of the second pole tip. The second pole tip should be well-defined in order to produce well-defined written tracks on the rotating disk. Poor definition of the second pole tip may result in overwriting of adjacent tracks. A well-defined second pole tip should have parallel planar side walls which are perpendicular to the ABS. This definition is difficult to achieve because the second pole tip is typically formed along with the yoke after the formation of the first insulation layer, the coil layer and the second and third insulation layers. Each insulation layer includes a hard-baked photoresist having a sloping front surface.
After construction, the first, second and third insulation layers present front sloping surfaces which face the ABS. The ZTH defining layer rises from a plane normal to the ABS at an angle (apex angle) to the plane. After hard baking of the insulation layers and deposition of a metallic seedlayer the sloping surfaces of the insulation layers exhibit a high optical reflectivity. When the second pole tip and yoke are constructed, a thick layer of photoresist is spun on top of the insulation layers and photo patterned to shape the second pole tip, using the conventional photo-lithography technique. In the photo-lithography light imaging step, ultraviolet light is directed vertically through slits in an opaque mask, exposing areas of the photoresist which are to be removed by a subsequent development step. One of the areas to be removed is the area where the second pole piece (pole tip and yoke) is to be formed by plating. Unfortunately, when ultraviolet light strikes the sloping surfaces of the insulation layers in a flaring region of the second pole piece, the ultraviolet light is reflected forward, toward the ABS, into photoresist areas at the sides of the second pole tip region. After development, the side walls of the photoresist extend outwardly from the intended ultraviolet pattern, causing the pole tip plated therein to be poorly formed. This is called "reflective notching". As stated hereinabove this causes overwriting of adjacent tracks on a rotating disk. It should be evident that, if the flare point is recessed far enough into the head, the effect of reflective notching would be reduced or eliminated since it would occur behind the sloping surfaces. However, this solution produces a long second pole tip which quickly reduces the amount of flux reaching the recording medium.
The high profile of the insulation stack causes another problem after the photoresist is spun on a wafer. When the photoresist is spun on a wafer it is substantially planarized across the wafer. The thickness of the resist in the second pole tip region is higher than other regions of the head since the second pole tip is substantially lower on the wafer than the yoke portion of the second pole piece. During the light exposure step the light progressively scatters in the deep photoresist like light in a body of water causing poor resolution during the light exposure step.
A scheme for minimizing the reflective notching and poor resolution problems is to construct the second pole piece with bottom and top second pole tips. The bottom second pole tip is constructed before the insulation layers to eliminate the reflective notching problem. After forming the first pole piece layer and the write gap layer, a photoresist layer is spun on the partially completed head. Ultraviolet light from the photo-patterning step is not reflected forward since the photoresist layer does not cover an insulation stack. Further, the photoresist is significantly thinner in the pole tip region so that significantly less light scattering takes place. After plating the bottom second pole tip the photoresist layer is removed and the first insulation layer, the coil layer and the second and third insulation layers are formed. The top second pole tip is then stitched (connected) to the bottom second pole tip and extends from the ABS to the back gap. Since the bottom second pole tip is well-formed, well-formed notches can be made in the first pole piece, as discussed hereinafter. However, with this head, the ZTH is dependent upon the location of the recessed end of the bottom second pole tip. Since the bottom second pole tip has to be long enough to provide a sufficient stitching area, this length may result in undesirable flux leakage between the first and second pole pieces. Since the top second pole tip is typically wider than the bottom second pole tip, the second pole piece has a T-shape at the ABS. The upright portion of the T is the front edge of the bottom second pole tip, and the cross of the T is the front edge of the top second pole tip. A problem with this configuration is that during operation, flux fringes from the outer corners of the top second pole tip to a much wider first pole piece at the ABS, causing adjacent tracks to be overwritten.
Once the bottom second pole tip is formed, it is desirable to notch the first pole tip of the first pole piece opposite the first and second corners at the base of the bottom second pole tip so that flux transfer between the pole tips does not stray beyond the track width defined by the bottom second pole tip. Notching provides the first pole piece with a track width that substantially matches the track width of the bottom second pole tip. A prior art process for notching the first pole piece entails ion beam milling the gap layer and the first pole piece, employing the bottom second pole tip as a mask. The gap layer is typically alumina and the first and second pole pieces and pole tips are typically Permalloy (NiFe). The alumina mills more slowly than the Permalloy; thus the top of the bottom second pole tip and a top surface of the first pole piece are milled more quickly than the gap layer. Further, during ion milling, there is much redeposition (redep) of alumina on surfaces of the workpiece. In order to minimize redep, the milling ion beam is typically directed at an angle to a normal through the layers, which performs milling and cleanup simultaneously. The gap layer in the field remote from the first and second corners of the bottom second pole tip is the first to be milled because of a shadowing effect at the first and second corners caused by the bottom second pole tip when the ion beam is angled. In this case, the ion stream will overmill the first pole piece before the gap layer is removed adjacent the first and second corners of the bottom second pole tip in the region where the notching is to take place. After the gap layer is removed above the sites where the notching is to take place, ion milling continues in order to notch the first pole piece. Overmilling of the first pole piece continues to take place in the field beyond the notches, thereby forming surfaces of the first pole piece that slope downwardly from the notches. As is known, such overmilling of the first pole piece can expose leads to the MR sensor, thereby rendering the head inoperative.
Even if overmilling of the first pole piece can be controlled, there is potentially a more troublesome problem, namely overmilling the top of the bottom second pole tip when the unwanted portions of the gap layer are milled and notches are formed. In order to compensate for this overmilling, the aspect ratio (ratio of thickness of photoresist to track width of the bottom second pole tip) is increased so that a top portion of the top of the bottom second pole tip can be sacrificed during the milling steps. When the aspect ratio is increased, definition of the bottom second pole tip is degraded because of the thickness of the photoresist, discussed hereinabove, resulting in track overwriting.
Another problem with the prior art merged MR head is that the profile of the MR sensor between the first and second gap layers is replicated through the second shield/first pole piece layer to the write gap layer causing the write gap layer to be slightly curved concave toward the MR sensor. When the write head portion of the merged MR head writes data the written data is slightly curved on the written track. When the straight across MR sensor reads this curved data there is progressive signal loss from the center of the data track toward the outer extremities of the data track.
All merged magnetic heads have a separation between the read and write gaps. This separation causes misregistration between the read and write gaps when the magnetic head is located at outer tracks on the magnetic disk. In the magnetic disk drive, an actuator swings the magnetic head across the rotating disk to various circular tracks on the disk. At the innermost track the read and write gaps are substantially aligned with one another and there is substantially no misregistration. At the innermost track the read gap follows within the track written by the write gap. However, when the actuator swings the magnetic head to the outermost track the read and write gaps are misaligned with respect to the track. If the write gap is within the track being written the read gap may be partially in the track and partially in an adjacent track. The misregistration increases with an increase in the separation between the read and write gaps. In magnetic heads where the write head is constructed before the read head the profile of the insulation stack of the write head raises the height of the first shield layer of the read head. It would be desirable if this profile could be reduced so that the read and write gaps are closer together.
Still another problem with prior art magnetic heads is that heating of high magnetic moment pole tips risks damage to the read sensor of the read head. A high magnetic material is Ni.sub.45 Fe.sub.55 as compared to Ni.sub.80 Fe.sub.20. Pole tips constructed of high magnetic material are desirable because they will conduct higher flux density without saturating. A still further problem with prior art magnetic heads is the risk of shorting of lead layers in the read head through the first read gap layer to the first shield layer. The first and second read gap layers are purposely very thin so as to narrow the read gap and increase linear bit density reading capability. Pinholes are more likely at steps in the first read gap layer than in flat portions of the first read gap layer. It is desirable that the partially completed magnetic head be planarized before the first read gap layer is constructed so as to reduce the chance of pinholes. The lack of planarity can cause still another problem in the construction of one or more coil layers. If there is a step, such as at a side edge of the first pole piece layer, this will cause reflective notching in adjacent portions of a photoresist layer employed to construct the coil layer.